Dehydration is the depletion of fluids and associated electrolytes from the body. Normally, a person's daily, total fluid amount is regulated to be within about ±0.02% of body weight, and water in the body may comprise approximately 63% of the entire body mass. A balance of bodily fluids is achieved and maintained by matching the input and excretion of liquid from the body, and an imbalance in fluids can be linked to either dehydration or hypohydration. Dehydration can be of particular concern for either the infirm, elderly, or infants, and can have serious consequences to a dehydrated person if not cared for properly. Loss of body fluids in amounts of less than about 2-5% body mass have been associated with reduced heat dissipation, loss of cardiovascular function, and decreased physical stamina.
Specific gravity of an individual's urine is a routinely measured means of evaluating the relative hydration status of the individual. Determination of urine volume and electrolyte concentrations can aid in monitoring whether the individual's body fluid amounts are in balance. Urine specific gravity (USG) refers to the ratio of the density of urine to the density of water. USG is affected mainly by the solids and ions in urine. USG correlates proportionally with the solid concentration and ion concentration of urine. USG normally ranges from 1.002 to 1.030. It is accepted that USG<1.020 is considered to be well hydrated, USG between 1.020 and 1.025 is considered to be semi-dehydrated and USG>1.025 is considered to be severely dehydrated. USG can be measured by an instrument such as either a urinometer or urine test dipsticks or strips. Modern dipsticks are commonly based on lateral flow assay technology. Three major methods, namely refractometry, hydrometry and reagent strips, are commonly used for USG measurements. Although refractometry and hydrometry are very accurate, they require special instruments and trained persons to operate.
Over the years, various manufacturers have attempted different methods to improve the performance of the dipsticks for specific gravity, such as different formulations to increase sensitivity and specificity. Problems, however, persist for all the commercially available dipsticks. A major problem is that the user has to read a change in color within a few brief minutes after dipping in the sample because the color development is not stable under test conditions. The signals that one may observe outside of the time window are often inaccurate, hence normally invalid. For some analyte tests, such as ion concentration in urine (i.e., specific gravity for dehydration), a certain time period is needed before a signal is fully developed and a valid reading can be achieved. This situation may not be a problem for a test that a user can constantly monitor; however, it becomes a problem when constant monitoring of the test is not feasible and sample introduction time is uncertain. For instance, it is difficult, if not impossible, to predict accurately when a baby or incontinent adult will urinate to provide a sample for an assay device in a diaper or other personal care product. Therefore, the assay device requires a validation mechanism to make sure that a reading is within the valid reading time window.
In recent years, reagent strips have become more popular, particularly in the over-the-counter and point-of-care markets, mainly due to their low cost and ease of use. In general, conventional reagent strips change color in response to the ionic strength of a urine sample. The ionic strength of urine is a measure of the amount of ions present in the urine. The USG is proportional to the ionic strength of the urine. Therefore, by assaying the ionic strength of the test sample, the USG can be determined indirectly and semi-quantitatively by correlating the ionic strength of the urine to the USG.
Conventional reagent strips are usually made in such a way that all the relevant reagents are diffusively immobilized together on a small porous zone on the strip. A sample of urine is then applied to the zone or the entire strip is dipped in the urine sample and then pulled out quickly to allow color to develop. Examples of such conventional reagent strips are described in U.S. Pat. No. 4,318,709 to Falb et al. and U.S. Pat. No. 4,376,827 to Stiso et al.
U.S. Pat. No. 4,318,709 to Falb et al. and U.S. Pat. No. 4,376,827 to Stiso et al., both of which are incorporated by reference herein, describe the polyelectrolyte-dye ion exchange chemistry utilized in conventional test strips for measuring USG. In such conventional test strips, ions present in urine induce an ion-exchange with a polyelectrolyte, thereby introducing hydrogen ions into the urine. The change in hydrogen ion concentration is detected by a pH indicator.
However, conventional reagent strips for USG measurement suffer from major shortcomings, particularly for over-the-counter and point-of-care markets. For instance, conventional reagent strips have a limited reading window because the signal produced by such strips begins to change only a short period of time after sample application. Signal change can be caused by reagent leaching (the result of diffusively immobilized reagents) and sample evaporation. Unless the strips are analyzed shortly after application of the sample, the signal change can lead to erroneous test results. Furthermore, because the reagents in conventional strips are typically water soluble, the strips must also be pulled out quickly from the urine sample to prevent the reagents from leaching into the sample. In addition, conventional reagent strips are often designed for only a single urine sample application. Multiple urine insults can lead to erroneous test results making such strips unsuitable for applications in absorbent articles where multiple urine insults cannot be controlled. Finally, conventional reagent strips do not provide a way for a user to know if the test has been performed correctly or if enough sample has been applied.
Although several kinds of dehydration dipsticks have existed for many decades, the existing technologies cannot meet the requirements for absorbent products. Some manufacturers of health and hygiene products have desired to broaden the appeal of their products to many consumers. These manufactures have a long felt desire to integrate dehydration sensors with absorbent products. This need has spurred development of low cost dehydration sensors that can be integrated into an absorbent article, such as a diaper or an incontinent garment. Nonetheless, previous lateral flow-based dehydration sensors that are feasible for incorporation into absorbent products are relatively complicated and economically inhibitive for integration of the sensors into every kind of absorbent garment. Thus, a need exists for an assay device that can provide such assurance to caregivers in a cost effective way.